Ores and the reactivity series
Definition
Ore
A rock that contains a metal or a metal compound in a large enough amount to make extracting the metal worthwhile.
- Most metals are found in the Earth's crust combined with other elements, often as metal oxides, in rocks called ores.
- Extracting a metal means separating it from its compound, which usually means removing oxygen from a metal oxide.
- How easy this is depends on the position of the metal in the reactivity series.
- Gold, at the bottom of the series, is so unreactive that it is found as the uncombined metal and only has to be separated from the surrounding rock.
- Metals below carbon, such as zinc, iron and copper, are extracted by heating their oxides with carbon, because carbon is more reactive and removes the oxygen.
- Metals above carbon, such as potassium, sodium, calcium, magnesium and aluminium, form compounds that carbon cannot break down, so they are extracted by electrolysis.
- The more reactive a metal is, the more strongly it holds on to the other elements in its compounds, so the harder and more expensive it is to extract.
- This is why gold and copper have been used for thousands of years, iron for about three thousand years, and aluminium only since the late 1800s, when electrolysis became possible on a large scale.
Practical
Extracting copper by heating with carbon
- Aim: to extract copper from copper(II) oxide by heating it with carbon, showing that carbon can remove oxygen from the oxide of a metal below it in the reactivity series.
- Apparatus: a hard-glass test tube, a bung fitted with a delivery tube, a second test tube containing limewater, a spatula, copper(II) oxide powder, powdered charcoal, a clamp and stand, a Bunsen burner, a heat-resistant mat and a beaker of cold water.
- Method, setting up:
- Mix one spatula of black copper(II) oxide with two spatulas of powdered charcoal on a sheet of paper, so that the carbon is in excess.
- Tip the mixture into the hard-glass tube and clamp the tube near its mouth.
- Fit the bung and arrange the delivery tube so that its end dips into the limewater.
- Method, heating:
- Heat the mixture strongly with a roaring Bunsen flame for about five minutes.
- Watch the limewater as gas bubbles through it.
- Lift the delivery tube out of the limewater before you stop heating, so that cold limewater cannot be sucked back into the hot tube.
- Let the tube cool, then tip its contents into the beaker of cold water and swirl, so that the light charcoal floats away and the dense copper sinks to the bottom.
- Results: the black mixture glows, pink-brown specks of copper are left in the tube, and the limewater turns milky, showing that carbon dioxide has formed.
- Reaction: 2CuO(s)+C(s)→2Cu(s)+CO2(g)\mathrm{2CuO(s) + C(s) \rightarrow 2Cu(s) + CO_2(g)}2CuO(s)+C(s)→2Cu(s)+CO2(g)
- Watch out: if magnesium oxide is heated with carbon in the same way, no magnesium forms, because magnesium is above carbon in the reactivity series.
- Safety: wear eye protection, remove the delivery tube from the limewater before you stop heating to prevent suck-back, and let the hard-glass tube cool completely before you touch it.
Raw materials for the blast furnace
Definition
Hematite
The main ore of iron, which contains iron(III) oxide.
- Iron is extracted from hematite, which is mainly iron(III) oxide, in a tall steel tower lined with heat-resistant bricks called a blast furnace.
- Three solid raw materials are added at the top of the furnace, and hot air is blasted in near the bottom.
- Hematite
- Hematite supplies the iron(III) oxide that is turned into iron.
- Coke
- Coke is almost pure carbon, made by heating coal.
- It is the fuel that heats the furnace and the source of the carbon monoxide that removes oxygen from the ore.
- Limestone
- Limestone is mainly calcium carbonate.
- It removes the sandy impurities in the ore, which are mostly silicon(IV) oxide, by turning them into slag.
- Hot air
- Hot air supplies the oxygen that the coke needs to burn.
Reactions in the blast furnace
Definition
Reduction (in terms of oxygen)
The loss of oxygen by a substance.
Definition
Slag
The molten waste, mainly calcium silicate, formed in the blast furnace when calcium oxide reacts with silicon(IV) oxide impurities in the ore.
- The reactions happen in a sequence as the solids move down the furnace and the hot gases rise up through them.
- Burning the coke
- Carbon in the coke burns in the hot air to produce carbon dioxide: carbon+oxygen→carbon dioxide\text{carbon} + \text{oxygen} \rightarrow \text{carbon dioxide}carbon+oxygen→carbon dioxide.
- This reaction is strongly exothermic, so it heats the bottom of the furnace to well over 1500 ∘C1500\ ^\circ\text{C}1500 ∘C.
- Making carbon monoxide
- The carbon dioxide rises and reacts with more hot coke to form carbon monoxide: carbon dioxide+carbon→carbon monoxide\text{carbon dioxide} + \text{carbon} \rightarrow \text{carbon monoxide}carbon dioxide+carbon→carbon monoxide.
- Carbon dioxide loses oxygen in this reaction, so it is reduced to carbon monoxide.
- Reducing the iron(III) oxide
- Carbon monoxide removes the oxygen from the iron(III) oxide to leave iron: iron(III) oxide+carbon monoxide→iron+carbon dioxide\text{iron(III) oxide} + \text{carbon monoxide} \rightarrow \text{iron} + \text{carbon dioxide}iron(III) oxide+carbon monoxide→iron+carbon dioxide.
- The iron(III) oxide is reduced, and carbon monoxide is the substance that reduces it.
- The furnace is so hot that the iron forms as a liquid, which trickles down and collects at the bottom.
- Decomposing the limestone
- The heat causes the thermal decomposition of calcium carbonate into calcium oxide and carbon dioxide: calcium carbonate→calcium oxide+carbon dioxide\text{calcium carbonate} \rightarrow \text{calcium oxide} + \text{carbon dioxide}calcium carbonate→calcium oxide+carbon dioxide.
- Forming the slag
- Calcium oxide reacts with the silicon(IV) oxide impurities to form slag, which is calcium silicate: calcium oxide+silicon(IV) oxide→calcium silicate\text{calcium oxide} + \text{silicon(IV) oxide} \rightarrow \text{calcium silicate}calcium oxide+silicon(IV) oxide→calcium silicate.
- Molten slag is less dense than molten iron, so it floats on top of the iron and the two liquids are tapped off separately.
- The solid slag is used in road building and for making cement.
- Waste gases, mainly nitrogen and carbon dioxide, leave through the top of the furnace.
Example
Following one lump of ore through the furnace
- A lump of hematite enters at the top of the furnace with coke and limestone.
- As it sinks, it meets hot carbon monoxide rising from the burning coke below.
- The carbon monoxide removes oxygen from the iron(III) oxide and leaves as carbon dioxide.
- The iron left behind melts, runs to the bottom of the furnace and is tapped off.
- The sandy impurities in the lump are removed by calcium oxide and leave the furnace as slag.
Extracting aluminium
Definition
Bauxite
The main ore of aluminium, which contains aluminium oxide.
Definition
Electrolysis
The breaking down of an ionic compound, when molten or in aqueous solution, by passing an electric current through it.
- The main ore of aluminium is bauxite, which contains aluminium oxide.
- Aluminium is above carbon in the reactivity series, so heating aluminium oxide with carbon does not produce aluminium.
- Aluminium is therefore extracted by electrolysis, in which a very large electric current breaks down the aluminium oxide.
- Electrolysis uses a huge amount of electrical energy, so aluminium plants are often built near cheap sources of electricity such as hydroelectric power stations.
- Iron is below carbon, so it can be extracted much more cheaply by heating with carbon in the blast furnace.
Exam technique
Answering extraction questions
- Give the job of each raw material, not just its name, so write that coke is a fuel and makes carbon monoxide, and that limestone removes impurities as slag.
- Name carbon monoxide as the substance that reduces iron(III) oxide.
- When asked why aluminium is not extracted with carbon, state that aluminium is more reactive than carbon.
- Use the word reduced for a substance that loses oxygen.
Common Mistake
- Do not say that iron is reduced, because it is the iron(III) oxide that is reduced to iron.
- Do not say that limestone reacts with the impurities directly, because it first decomposes into calcium oxide, which forms the slag.
- Do not confuse the ores, because hematite is the ore of iron and bauxite is the ore of aluminium.
Self review
- Why are metals high in the reactivity series harder to extract?
- Name the three solid raw materials added to the blast furnace.
- Which substance reduces iron(III) oxide in the blast furnace?
- What is the job of limestone in the blast furnace?
- What is the main ore of aluminium, and how is aluminium extracted?
